This invention relates to a deflection synchronizing arrangement of a television apparatus
In a television receiver, the electron beams inside the television receiver picture tube are deflected by magnetic fields generated by sawtooth deflection currents flowing in horizontal and vertical deflection windings. The deflected electron beams scan a raster pattern on the picture tube phosphor screen. The raster pattern, without correction, may display various geometric distortions such as side or east-west pincushion distortion. To provide side or East-West (E-W) pincushion distortion correction or E-W pin distortion correction, the peak-to-peak horizontal deflection trace current in the horizontal deflection winding is modulated at a vertical rate in a parabolic manner, for example, by using a diode modulator.
FIG. 1a illustrates the raster pattern of a pair of vertical curved lines, 300 and 301, that are ideally straight, displayed on a large size cathode ray tube (CRT) having a flat screen with uncorrected differential pin distortion and uncorrected pin distortion. Without correction, the raster pattern of vertical line 300 at the extreme left of a CRT screen 304 will have a different amount of east-west pincushion distortion than vertical curved line 301 at the extreme right of CRT screen 304. As manifested by distance parameters d300 and d301, a larger amount of pincushion distortion appears at the left, or start of scan edge, than at the right of the screen. Such distortion is referred to herein as a differential pin distortion.
The large vertical rate modulation of horizontal deflection current required for E-W pin distortion correction on a flat CRT can interact with the linearity coil in such a way as to result in an increased differential pin. When differential pin distortion exists, the diode modulator, by itself, may not be capable of fully correcting such distortion because the diode modulator operates symmetrically with respect to a vertical line 310 at the screen center.
In order to synchronize the scanning of the beam with the display video information, the horizontal drive signal of the horizontal deflection circuit output stage is synchronized with a stable horizontal synchronizing signal contained in a composite video signal. When transmitted, the synchronizing signal pulses recur at, for example, 15,625 Hz that is stable. However, when received, the synchronizing signal may no longer be stable and may contain distortions in the form of signal noise. Because of the presence of noise, synchronization of the horizontal deflection circuit with the horizontal synchronizing signal pulses is obtained in a well known manner by the use of a voltage controlled oscillator (VCO) that is included in a phase-lock loop circuit (PLL). The VCO generates a signal at a frequency that is equal to, for example, a high multiple of the frequency of the synchronizing signal. The VCO generated signal is frequency divided down in a frequency divider and an output signal approximately equal to the horizontal frequency is generated. The frequency divider output signal and the synchronizing signal are coupled to a phase detector having an output signal that is coupled to a loop filter having a relatively long time constant. An output signal of the filter is coupled to a frequency/phase control terminal of the VCO for controlling the frequency/phase of the VCO output signal.
When a synchronizing pulse is obscured by noise, the rate of the VCO, nevertheless, remains substantially unchanged by the PLL operation and the deflection circuit continues to receive regular deflection control pulses. A frequency divider arrangement produces from the output signal of the VCO a horizontal-rate output signal with high stability. The horizontal rate output signal may be phase locked by the PLL to the average phase of the incoming synchronizing signal.
The timings of a deflection current in a deflection winding and of the retrace pulses produced by the horizontal deflection circuit output stage may vary in a manner dependent upon various factors such as transient loads on the flyback transformer due to audio or video transients, which in turn modulate the turnoff time of the horizontal output transistor. This variation in the timings of the horizontal retrace pulses, disadvantageously, may cause a distortion of the displayed image. To prevent the occurrence of a variation in a delay of the deflection current relative to the synchronizing signal, it is known to utilize additionally a phase-control loop circuit (PCL) to form a dual feedback loop arrangement.
The PCL includes a second phase detector having a first input terminal that is responsive to an output signal of the VCO of the PLL and a second input terminal that is responsive to retrace pulses produced in the horizontal deflection circuit output stage. The second phase detector produces a phase difference indicative signal that is coupled to a loop filter of the PCL. A controllable phase shifter is responsive to an output signal generated by the PCL loop filter for producing horizontal-rate drive pulses at a dynamically controllable phase shift. Thereby, the horizontal-rate drive pulses are maintained in a constant phase relationship with the output signal of the PLL.
It is known to correct differential pin distortion by phase shifting, in the PCL, the phase of a horizontal drive signal of the horizontal deflection circuit output stage. Thereby, elimination of the left edge pincushion distortion of curved line 300 of FIG. 1a to form straight line 300′ FIG. 1b and elimination of right edge pincushion distortion curved line 301 of FIG. 1a to form straight line 301′ FIG. 1b is obtained. Consequently, a bow line 310′ is obtained instead of straight line 310 of FIG. 1a. Similar symbols and numerals in FIGS. 1a and 1b indicate similar items or functions.
Synchronization of a horizontal deflection circuit with horizontal synchronizing signal pulses is obtained by the use of a VCO that is included in a PLL. A PCL includes a phase detector having a first input terminal that is responsive to an output signal of the VCO and a second input terminal that is responsive to retrace pulses of a horizontal deflection circuit output stage. Correction of differential pin distortion is accomplished by dynamically phase shifting the phase of the output signal of the PLL in a vertical rate parabola manner. A vertical rate parabola signal provides phase shifting of the horizontal drive signal of the horizontal deflection circuit output stage in a vertical rate parabola manner. By phase shifting the phase of the output signal of the PLL, the horizontal drive signal of the PCL is less susceptible to noise than if phase shifting occurs downstream of the PLL loop filter. This is so because the vertical rate parabola signal is inserted upstream of the loop filter of the PLL.